reconsider A = C, B = D as Subset of T ;
A1: A is open by Th2;
A2: B is open by Th2;
A3: A is closed by Th3;
A4: B is closed by Th3;
A \/ B is open by A1, A2, TOPS_1:37;
hence C \/ D is Element of OpenClosedSet T by A3, A4, Th4, TOPS_1:36; :: thesis: verum